Loading Multiple Text into Dynamic Text Field

I have several checkboxes that I would like to display their values in a dynamic text field, adding to what is previously there instead of replacing it. I also need the value to disappear from the text field if the checkbox is unchecked.

So far, I have (on each checkbox):

on(click) {
	_root.textfield.text = this.label;
}

On the first frame of my form, I have the following for those checkboxes:

_root.checkbox.addListener();

What step do I need to take next?